Reverse osmosis membrane (RO membrane) methods have hitherto been widely used in a desalination method of seawater. A reverse osmosis desalination method (RO method) is a method in which a pressure of about 55 atmospheres is applied to an osmosis membrane in a direction opposite to the osmotic pressure, thereby taking out fresh water from about 3.5% by weight seawater.
It is known that when a polymer electrolyte membrane obtained by a direct graft-polymerization of an electrolyte monomer to a hydrophobic polymer membrane is utilized in the RO method, a transmembrane flow rate is increased.
On the other hand, a forward osmosis membrane seawater desalination method (FO method) is known as the desalination method. According to this method, an osmosis membrane, which is the same as that used in the RO method, is used, and an aqueous ammonium carbonate having a higher concentration than that of seawater is disposed at a support membrane side, whereby fresh water is drawn into the aqueous ammonium carbonate side due to an osmotic pressure caused by ammonium carbonate, without applying a pressure. After that, the temperature of the ammonium carbonate solution is elevated to about 60° C. by heating it to decompose it into carbonic acid and ammonia, from which water is removed, thus resulting in acquisition of fresh water.
Further, there is also a composite semipermeable membrane having an ionic group or non-ionic group.